The present paper is a part of the rejoinder to Nakagaki's 1987 criticism against Sayeki's concept of feeling of "indeed" in deep understanding, Nakagaki (1987) argued that mental models proposed by Sayeki for understanding (1) the area of a parallelogram, (2) the direction of spool movement, and (3) pully movement were all not only full of contradictions but also were not supported by any experimental evidence and that a feeling of "indeed" would not guarantee true understanding. The present paper shows that the mental model by use of of a bundle of paper for understanding the area of parallelogram is quite consistent with the mathematical concept of area in terms of integral caluculus, and also quite successful in teaching its formula. The rejoinder to the other points, i, e., (2) and (3), are not included in this paper (to be published in Studies in Cognitive Precesses; 1988 Reports of Sayeki Seminar on Cognitive Process, Facuty of Education, University of Tokyo), This paper focuses upon the development of the concept of "mental models" in cognitive psychology, and its relation to the philosophical concept of the fixation of belief (practically equivallent to feeling of "indeed") in epistemology,